1. Field of the Invention
The present invention relates to a color photographing device such as an electronic camera, a digital video camera or the like. Specifically, the present invention relates to a white balance adjustment carried out by a color photographing device.
2. Description of the Related Art
In general, color temperature of a light source for illuminating an object is different among photographing places. Colors reproduced as an image reflect colors of the object itself and colors of the light source for illuminating the object. Therefore, white balance is adjusted in the color photographing device so that a white object is reproduced to be white (for example, see Japanese Unexamined Patent Publication No. 2000-92509 and Japanese Unexamined Patent Publication No. Hei 7-193826).
For example, in a color photographing device in which a red pixel signal R, a green pixel signal G and a blue pixel signal B are output by an image sensor, white balance is adjusted so that the signal levels of R, G and B are equal to one another when achromatic color is photographed. Specifically, the color temperature of a light source is first estimated. Then, R and B are multiplied by gains Wr and Wb, respectively, for white balance processing (hereinafter referred to as “white balance gains”) which correspond to the color temperature.
FIG. 1 shows an example of a spectral sensitivity characteristic of a conventional image sensor, which is normalized on the basis of the peak value of the sensitivity of green pixels, and FIG. 2 shows a white balance gain characteristic to the image sensor having the spectral sensitivity characteristic shown in FIG. 1. As is apparent from FIG. 2, the conventional image sensor has a spectral sensitivity characteristic that the ratio of the signal levels of R, G and B are equal to about 0.45:1:0.55 when achromatic color illuminated by a light source having color temperature of 5000K is photographed. The following two reasons can be given as a part of the reasons that the spectral sensitivity characteristic of the image sensor has been set as described above.
First, G contains a larger mount of brightness information than R or B. Therefore, when the signal level of G is varied, the brightness of an overall image is varied.
Accordingly, when white balance is adjusted, the white balance gain Wg to G must be set to 1.
Second, the white balance gains Wr and Wb are normally required to be equal to 1 or more. This is because if there is any color temperature at which the white balance gains Wr and Wb are set at less than 1, the highlight becomes colored at the color temperature.
More specifically, if Wr, Wb>1, pixel signals R, G, and B are held saturated, and thus the highlight remains white. However, if white balance is adjusted with Wr being smaller than 1, G and B remain at their maximum values, but R is set to be smaller than the maximum value thereof. Accordingly, the white of the highlight is reproduced to be slightly blue-green.
In the conventional color photographing device, as shown in FIG. 2, about double the white balance gain was applied to both Wr and Wb when color temperature was around 5000K (color temperature at which photographing is considered to be most frequently carried out). However, the higher the Wr and Wb are, the more remarkable the troubles are, such as increased noises of R and B and discontinuity of gradation. Particularly, color noises can be remarkable at dark portions of an image.